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Aero Question..........

 
Old 02-10-2010, 03:53 AM
  #46  
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most of the function of the diffuser will be in area of drag, not nessarily downforce.

You said there was claims that the diffuser provided downforce values?

Interesting.



mk

Originally Posted by wanna911 View Post
There are some manufacturers that are making rear diffusers for 911's at stock height and showing transition from lift to DF at various speeds. Do you guys think this is a bunch of fluff???






Hard to argue with wind tunnel testing eh? I can't find the graph of the claimed DF numbers though. But this diffuser doesn't even cover the whole bottom of the rear.
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Old 02-10-2010, 03:59 AM
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got it!

Hey, the old wives tail of 10% loss for scotch tape has been around for a feew years. Its clearly BS and easily proved otherwise several ways, even if you dont have a wind tunnel. usually the tail was a piece of scotch tape on the top of the wing. sure, you want the surface to be smooth, but that cant change the L/D ratio by 10%.

Originally Posted by claykos View Post
That's why I said as big a wing AND a splitter as practical - I said you couldn't have too much downforce, not taht you couldn't have too much rear downforce.

Also thought I would share an interesting point to somewhat illustrate the futility of really trying to optimize aero within the frame of club racing. I happened to listen to a presentation this afternoon about some wind tunnel experiments on airfoils. A piece of scotch tape placed on the leading edge of the airfoil made close to a 10% difference in Lift/Drag ratio....

Not to say people shouldn't add aero to their cars, it helps - A LOT! But, without serious testing and engineering quibbling over a few extra pounds of drag force, etc is not worth it. So....if the rules allow run a big *** wing and splitter. If the balance is off try adding downforce to fix it - if you can't, then remove downforce from the offending end of the car.
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Old 02-10-2010, 04:04 AM
  #48  
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Jack, that is a great shot! think that was caused by the diffuser??

straight line numbers at Willow, I believe!

Did the car get hurt?

mk

Originally Posted by JackOlsen View Post
Yeah, so long as you never have to turn, it's awesome.

I got better straight line numbers with this.





But it didn't help my lap times any.

Made the car a little less stable, in fact:

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Old 02-10-2010, 08:20 AM
  #49  
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Tom the rear bumper shot was big help. Thanks. I did the center cut out, but did not consider the side ducting too. Now I know how to model it. I have a spare ripped up bumper to practice on first. The only issue I am not sure of is reforming the rear wheel wells to minimize the air spill and tire debris continuing backwards. I guess braking cooling should be considered also.
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Old 02-10-2010, 09:15 AM
  #50  
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Originally Posted by claykos View Post
A piece of scotch tape placed on the leading edge of the airfoil made close to a 10% difference in Lift/Drag ratio....
Actually the tape in question is thicker - helicopter tape. I have been told, by 2 different F1 aerodynamicists that this is correct.

But, you can expect the pretend experts here to come in and jump on this - just watch. "It is impossible, I can prove t with math, my friend who works with wings but has never worked with aero on a car, blah, blah, blah." I never could understand why that person or two don't run the aero department of an F1 team - LOL.

You will figure out quickly who they are and who not to listen to.
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Old 02-10-2010, 09:20 AM
  #51  
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Just in general on the diffusers and bottoms...

From the photos, the carbon diffuser is going on a stock bottom. That is covered with ridges, holes, significant overlaps, etc. The airflow is going to be pretty turbulent so smoothing the exit with the 'diffuser' doesn't change things. With Jack's car, the bottom appears pretty smooth. The diffuser should help with drag as it smooths the flow under the engine area. While it won't do much in the way of downforce, because of ride height, it will help keep air from 'collecting' at the front of the car and at the under-engine area and that will reduce drag.
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Old 02-10-2010, 09:24 AM
  #52  
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Originally Posted by mark kibort View Post
most of the function of the diffuser will be in area of drag, not nessarily downforce.

You said there was claims that the diffuser provided downforce values?

Interesting.



mk
So you are saying that a rear diffuser reduces drag but does not produce DF? That is similar to what I've read though the diffuser does produce small amounts of DF with far less of an increase in drag compared to a rear wing.

How about the Enzo though, Ferrari claimed some pretty insane DF numbers for that car with no rear wing at all. Like 1700 lbs which is equal to an ACR albeit at a much higher speed.

Originally Posted by SundayDriver View Post
Just in general on the diffusers and bottoms...

From the photos, the carbon diffuser is going on a stock bottom. That is covered with ridges, holes, significant overlaps, etc. The airflow is going to be pretty turbulent so smoothing the exit with the 'diffuser' doesn't change things. With Jack's car, the bottom appears pretty smooth. The diffuser should help with drag as it smooths the flow under the engine area. While it won't do much in the way of downforce, because of ride height, it will help keep air from 'collecting' at the front of the car and at the under-engine area and that will reduce drag.
So then a diffuser used in this scenario would be good, especially on high speed tracks to reduce drag created by big top wing thereby allowing you to run more angle on said wing wit less increase in drag.

I guess the question then would be how significant is the drag reduction by the diffuser compared to the drag increase by the angle of attack in the wing.
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Old 02-10-2010, 10:19 AM
  #53  
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Folks, I have nothing substantial to add to this conversation as I am certainly not qualified in this area of science but I am learning alot from the discussion so thank you...

As most of you know a modern street Porsche has a plastic undertray to supposedly help with aero etc but is also has scoopes and holes so I'm not sure how much it actually benefits but it is one of the cleanest produced street cars out there.

A couple of years ago I had the opportunity to go to Maranello where they had Schumacher's car on a glass turntable that you could walk underneath and look at. The bottom of the car was as smooth and slick as the top was. Very impressive.

Carry on...
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Old 02-10-2010, 11:32 AM
  #54  
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why would it increase downforce? I guess, due to its shape in the rear, you could get a slight drop in presure, which would increase downforce (or reduce the pressure on the bottom, in that area, to change the pressure differential which would increase downforce slightly). and yes, you are right, this downforce comes at no drag cost, similar to the concepts of the splitter in the front.

Now, ferrarri??? A whole 'nuther set of concepts. think venturi tunnels under the car. that can suck the car right to the road. (very low pressure , and that pressure change is directly proportionate to downforce).

Originally Posted by wanna911 View Post
So you are saying that a rear diffuser reduces drag but does not produce DF? That is similar to what I've read though the diffuser does produce small amounts of DF with far less of an increase in drag compared to a rear wing.

How about the Enzo though, Ferrari claimed some pretty insane DF numbers for that car with no rear wing at all. Like 1700 lbs which is equal to an ACR albeit at a much higher speed.



So then a diffuser used in this scenario would be good, especially on high speed tracks to reduce drag created by big top wing thereby allowing you to run more angle on said wing wit less increase in drag.

I guess the question then would be how significant is the drag reduction by the diffuser compared to the drag increase by the angle of attack in the wing.
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Old 02-10-2010, 11:49 AM
  #55  
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Mark, We have gone over this before. now the tape is thicker. before it was on the top of the wing, (flat surface) now the wives tail has changed to the leading edge. Com'mon, get real. you have 2 different aero engineers saying this, and 1 F1 aero player? Maybe you can pretend that you really understood what they were talkiing about

Look, the wing surface is a complicated thing. you are saying tape along the leading edge, on the wing, etc etc, is going to change the Lift to Drag ratio by 10%. thats HUGE, and totally unrealistic. Jack can disprove this very easily with his sensors and so can I. I mean , a million flaws are part of that theory. First off, at what angle of attack what kind of airfoil, what shape? and of course, where is the tape stuck? sure, if itis on the bottom of the air foil, it can kill lift, but not on the top. Look at airplane de icing. they only clean the top and the leading edge of the wing to get the laminar flow back so the wing produces lift efficiently. Now, the leading edge of an "ice'ed" wing, is not smooth like tape, but i suppose on the leading edge, that could effect L/D ratios, but not on the the racing wing bottom where this claim originally started from.

To give you an idea and scale to show how far fetched this is. putting a thin layer of tape on the leading edge or botttom of our wings, could change downforce by over 10% for a given angle. (i.e. if im making 200lbs of downforce at 10 degrees wing angle, and producing 20lbs of drag, this tape will have to change the downforce to 180lbs)

I could be wrong, as i dont know what the thickness of the tape being mentioned is and many of the details, but at first glance, you dont need to worry about the wing surface that much. its not like we are flying 600mph airplanes. we are talking 60-130mph wings



Originally Posted by SundayDriver View Post
Actually the tape in question is thicker - helicopter tape. I have been told, by 2 different F1 aerodynamicists that this is correct.

But, you can expect the pretend experts here to come in and jump on this - just watch. "It is impossible, I can prove t with math, my friend who works with wings but has never worked with aero on a car, blah, blah, blah." I never could understand why that person or two don't run the aero department of an F1 team - LOL.

You will figure out quickly who they are and who not to listen to.
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Old 02-10-2010, 11:52 AM
  #56  
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Originally Posted by SundayDriver View Post
Actually the tape in question is thicker - helicopter tape. I have been told, by 2 different F1 aerodynamicists that this is correct.

But, you can expect the pretend experts here to come in and jump on this - just watch. "It is impossible, I can prove t with math, my friend who works with wings but has never worked with aero on a car, blah, blah, blah." I never could understand why that person or two don't run the aero department of an F1 team - LOL.

You will figure out quickly who they are and who not to listen to.
The experiment I'm talking about was with scotch tape. The experiments were performed by a friend/coworker - I literally listened to the results yesterday - I'm not talking about internet myths or wives tales! Scotch tape at the leading edge reduced glide ratio (L/D) by ~10% percent over the smooth wing (for this particular airfoil - the experiments were trying to test a configuration that was claimed in literature to enhance performance, but so far the results could not be duplicated). This change in performance was primarily at the angle of attack providing max L/D - the effect was negligible at other angles of attack. A piece of thicker packing tape placed a mid-chord had little effect on performance.

I'm not talking about changing the entire L/D of a vehicle of 10% - I am talking about JUST the airfoil in wind tunnel conditions. The drag created by this airfoil would be minute in the overall sense of a race car.

Mark K: READ CAREFULLY: I did not say it changed downforce by 10% - I said it changed Lift/Drag by 10%.

The physics behind it has to do with changing the location of a shallow region of separation on the suction side of the airfoil. I looked at the flow vis pictures yesterday - the leading edge tape changes the separation/reattachment point of the flow a NOTICABLE amount. It's not about a smooth or rough surface - a small change in the right location of the airfoil (which will be different on different airfoils) will have LARGE impacts on the location of separation and reattachment. This separation bubble effectively changes the shape of the airfoil because it is a region of recirculated flow that the general flow is traveling above.

Also note that on the back of a race car with highly turbulent approach flow the result might be completely different. Sorry, I trust controlled wind tunnel experiments with precise and very expensive measuring and flow vis equipment more than some guy with a 928 :-)

Last edited by claykos; 02-10-2010 at 12:51 PM. Reason: added some info on separation
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Old 02-10-2010, 12:15 PM
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I would certainly believe the effect on lift. Modern aircraft have crashed due to wing icing which I believe the speeds are as low as 160-180mph. Looks like my best option is the 60inch wing with side plates and I will also add the flush rear window kit which the newer 997/996 platforms have by design. My mirrors are the integrated EVO doors so profile that is minimal. What about the front hood air inlet on the stock setup. Any reason not to get rid of the intake with an insert? I also take the wipers off in the dry as they really drag a lot. Anyone bother with a light insert that better follows the hood angle? The 993 lights are only slightly better than the 964 upright profile.
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Old 02-10-2010, 12:38 PM
  #58  
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Originally Posted by claykos View Post
The experiment I'm talking about was with scotch tape. The experiments were performed by a friend/coworker - I literally listened to the results yesterday - I'm not talking about internet myths or wives tales! Scotch tape at the leading edge reduced glide ratio (L/D) by ~10% percent over the smooth wing (for this particular airfoil - the experiments were trying to test a configuration that was claimed in literature to enhance performance, but so far the results could not be duplicated). This change in performance was primarily at the angle of attack providing max L/D - the effect was negligible at other angles of attack. A piece of thicker packing tape placed a mid-chord had little effect on performance.

I'm not talking about changing the entire L/D of a vehicle of 10% - I am talking about JUST the airfoil in wind tunnel conditions. The drag created by this airfoil would be minute in the overall sense of a race car.

Mark: READ CAREFULLY: I did not say it changed downforce by 10% - I said it changed Lift/Drag by 10%.

The physics behind it has to do with changing the location of a shallow region of separation on the suction side of the airfoil. I looked at the flow vis pictures yesterday - the leading edge tape changes the separation/reattachment point of the flow a NOTICABLE amount. It's not about a smooth or rough surface - a small change in the right location of the airfoil (which will be different on different airfoils) will have LARGE impacts on the location of separation and reattachment. This separation bubble effectively changes the shape of the airfoil because it is a region of recirculated flow that the general flow is traveling above.

Also note that on the back of a race car with highly turbulent approach flow the result might be completely different. Sorry, I trust controlled wind tunnel experiments with precise and very expensive measuring and flow vis equipment more than some guy with a 928 :-)
Sorry - Missed the L/D part. Yes, it does not take much to change the separation point on a wing and that has a huge effect. I guess I was referencing something different, which is the loss of downforce with helicopter tape.
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Old 02-10-2010, 12:46 PM
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Originally Posted by SundayDriver View Post
Sorry - Missed the L/D part. Yes, it does not take much to change the separation point on a wing and that has a huge effect. I guess I was referencing something different, which is the loss of downforce with helicopter tape.

Your reference showed similar results though - loss of performance with the leading edge tape!

And I think I'm done with this thread now...
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Old 02-10-2010, 12:52 PM
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Well, there you go . conditions. yes, give all the details and we can see if it is possible.

glide ratio is not lift to drag of the wing

you said, " Mark: READ CAREFULLY: I did not say it changed downforce by 10% - I said it changed Lift/Drag by 10%."

maybe its early in the AM, but isnt that the same thing

200lbs downforce , 20lbs of drag = 10:1 lift to drag
lift to drag changing to 10% less, or going to 9:1
180lbs (down 10% or 20lbs and drag still 20bs)

point is, the claim is substantial. for our wings, with naca numbers similar to the one Ive shown from one of my aero texts, it would be very unlikely that a piece of scotch tape could cause a 10% change in Lift/drag ratio.

originally, it was on the flat side of the wing, from what I remember. and I brought up the point of most early wings not even having "bottom" of wings. only the tops to fly effectly.

as you said, it was at only one particular angle of attack. sure, this is possible. it could be anywhere. extreme, and unique situations or conditions doesnt help the discussion here. we dont run our wings at max L/D ratios and all other points were "negligible". so, the claim has little or no point or value, does it?




Originally Posted by claykos View Post
The experiment I'm talking about was with scotch tape. The experiments were performed by a friend/coworker - I literally listened to the results yesterday - I'm not talking about internet myths or wives tales! Scotch tape at the leading edge reduced glide ratio (L/D) by ~10% percent over the smooth wing (for this particular airfoil - the experiments were trying to test a configuration that was claimed in literature to enhance performance, but so far the results could not be duplicated). This change in performance was primarily at the angle of attack providing max L/D - the effect was negligible at other angles of attack. A piece of thicker packing tape placed a mid-chord had little effect on performance.

I'm not talking about changing the entire L/D of a vehicle of 10% - I am talking about JUST the airfoil in wind tunnel conditions. The drag created by this airfoil would be minute in the overall sense of a race car.

Mark: READ CAREFULLY: I did not say it changed downforce by 10% - I said it changed Lift/Drag by 10%.

The physics behind it has to do with changing the location of a shallow region of separation on the suction side of the airfoil. I looked at the flow vis pictures yesterday - the leading edge tape changes the separation/reattachment point of the flow a NOTICABLE amount. It's not about a smooth or rough surface - a small change in the right location of the airfoil (which will be different on different airfoils) will have LARGE impacts on the location of separation and reattachment. This separation bubble effectively changes the shape of the airfoil because it is a region of recirculated flow that the general flow is traveling above.

Also note that on the back of a race car with highly turbulent approach flow the result might be completely different. Sorry, I trust controlled wind tunnel experiments with precise and very expensive measuring and flow vis equipment more than some guy with a 928 :-)
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